Embodiments of inventive concepts relate to semiconductor devices and methods of fabricating the same, and more particularly, to semiconductor devices having metal gates and methods of fabricating the same.
A field effect transistor (hereinafter, referred as to “transistor”) is a significant element of semiconductor device. In general, a transistor has source and drain regions, which are formed spaced apart from each other in a semiconductor substrate, and a gate electrode covering a channel region between the source and drain regions. The source and drain regions may be formed by implanting dopants into the semiconductor substrate. The gate electrode may be electrically separated from the channel region by a gate insulating layer between the semiconductor substrate and the gate electrode. In semiconductor devices, transistors are widely used as switching devices and/or elements of logic circuits.
Operating speeds of semiconductor devices are increasing. Sizes of transistors are decreasing, increasing integration densities of semiconductor devices, and this may lead to reductions in turn-on currents of transistors, which may reduce transistor speeds. In addition, the reduction in sizes of transistors may lead to increases in contact resistances between source or drain regions and contact-plug structures, which may be another factor reducing transistor speeds. Owing to these factors, it may be increasingly difficult to satisfy increasing demands for fast operating speeds of semiconductor devices. To increase speeds of transistors, gate electrodes may be formed to have reduced resistance. For example, gate electrodes may be formed to include metal layers of reduced resistivity.